To produce semiconductor elements, ultra high purity nitrogen having a very low content (for example below 1 Vppm) of reducing components such as hydrogen and carbon monoxide is required. It is known, for example, in the preparation of such ultra high purity nitrogen, to remove impurities such as carbon monoxide and hydrogen from the intake air of an air fractionation plant by catalytic oxidation of carbon monoxide to carbon dioxide and/or hydrogen to water on catalyst beds using solid catalysts. The catalyst material can consist of hopcalite (mixture of oxides and/or peroxides of manganese, silver, cobalt and/or copper), or of a bed of aluminium oxide, in/on which palladium or platinum or else ruthenium, osmium, iridium or rhodium is incorporated or applied. The catalyst bed can be disposed directly downstream of the air compressor and can be operated at an elevated temperature of about 160.degree. C. As an alternative thereto, it is possible to dispose the catalyst bed in the vessel in which the molecular sieve for drying the air is also situated.
Examples of the purification of feed air for an air fractionation plant for generating an ultra pure gas are found in EP-0 438 282-A1 and EP-0 454 531-A1. According to EP-0 438 282-A1, water is initially removed from the feed air stream by adsorption; thereafter, carbon monoxide and hydrogen are converted at ambient temperatures of 0 to 50.degree. C. in one layer each of hopcalite and palladium-activated aluminium oxide and finally residual water, carbon dioxide and hydrocarbons are removed by adsorption. Using the known processes, typically, depletions of carbon monoxide and hydrogen by a factor of 20 are possible.